1. Field of the Invention
This invention relates to a process for forming a polycrystalline silicon film, usually referred to as polysilicon film, which is suitably used in semiconductor devices, especially in channels of thin film transistors (TFTs).
2. Related Background Art
In forming a polycrystalline silicon film on a silicon wafer for the fabrication of a TFT, conventionally an amorphous silicon film is deposited on the surface of a silicon oxide film as a substrate covering a silicon wafer, continuously without any treatment step intervening, and next annealed for crystallization, whereby a polycrystalline silicon film for the channel of the TFT is formed.
FIGS. 1 to 3 show the steps of forming the polycrystalline silicon film on the silicon oxide film as described above. First as in FIG. 1, a silicon wafer 1 is covered in an about 600 nm-thickness silicon oxide film 2 (SiO.sub.2) as the substrate, and then immediately an about 50 nm-thickness amorphous silicon film (a-Si) 3 is deposited on the silicon oxide film 2 as in FIG. 2.
Then, the wafer is loaded into an annealing furnace to be annealed at 600.degree. C. for, e.g., about 3 hours, whereby the amorphous silicon film 3 is crystallized, and as in FIG. 3, a polycrystalline silicon film 4 is formed on the silicon oxide film 2.
The technique for depositing the amorphous silicon film 3 on the silicon oxide film 2 is generally a known technique, such as low-pressure CVD(Chemical Vapor Deposition), vacuum deposition, or others. One of the significant problems in fabricating a practical TFT is that the off current of the TFT is stably retained below 0.25 pA/gate, i.e., the standby current is retained below 1 .mu.A.
As processes for forming a polycrystalline silicon film which can solve this problem, the following two processes, for example, are presented in "NIKKEI MICRODEVICES" (March, 1990. pp. 64-71).
In a first one of these processes, crystal grain diameters of a polycrystalline silicon film to provide a channel are increased so as to reduce grain boundaries through which an off current flows. The above-mentioned amorphous silicon film is deposited at a relatively low temperature (480.degree.-550.degree. C.) so as to lower a generation rate of crystalline nuclei. In a second process, grain diameters of a polycrystalline silicon film to provide a channel are made uniform to a suitable extent, and the film thickness of the polycrystalline silicon film is decreased, whereby a reduced off current is obtained.
The above-described reference describes especially that disilane (Si.sub.2 H.sub.6), whose activation energy is low, is used in place of silane (SiH.sub.4) gas for the fabrication of TFTs, and the deposition temperature of an amorphous silicon film is set at about 480.degree. C., whereby a polycrystalline silicon film of an about 3 .mu.m-grain diameter can be formed (the first process).
In the conventional processes for forming a polycrystalline film, as described above, since the deposition of an amorphous silicon film and the annealing thereof for the crystallization immediately following the formation of a silicon oxide film causes a state in which crystalline nuclei tend to be generated, in the annealing, at the interface between the silicon oxide film and the amorphous silicon film takes place, a number of crystalline nuclei are generated in the annealing, with the result that a crystal grain diameter after the crystallization is suppressed to about 1 .mu.m at largest.
Furthermore, although it is possible to form a polycrystalline silicon film of large grain diameters by the conventional process for forming a polycrystalline silicon film as described in the above reference (the first process), the inventors confirmed by comparison tests that an amorphous silicon film is deposited in a film thickness of 50 nm by a low-pressure CVD apparatus under the conditions of Si.sub.2 H.sub.6 gas, 480.degree. C. and a film forming speed of 1 nm/min, and annealed at 600.degree. C. for 3 hours (the usual annealing time), whereby the resultant polycrystalline silicon film can have a crystal grain diameter of about 1 .mu.m, and to grow the crystal grain diameter up to about 3 .mu.m, it is necessary to form the polycrystalline silicon film at a speed above 5 nm/min.
Accordingly, the process for forming a polycrystalline silicon film described in the above reference can form a polycrystalline silicon film of a large grain diameter. But because of its high film forming speed, the film thickness uniformity of the polycrystalline silicon film is .+-.3% at a film forming speed of 1 nm/min, but at a film forming speed above 5 nm/min the uniformity is adversely above 10%.